Why Collector Urchins Carry Shells and Seaweed on Their Backs

On a shallow Indo-Pacific reef, look for a shell fragment perched on a dome of short spines, with one edge creeping uphill. That movement gives away a collector urchin at work: the shell hasn’t grown there, and it hasn’t simply fallen into place.

The collector urchin, Tripneustes gratilla, grips loose pieces with its tube feet and moves them over its body with the help of its spines. A piece held overhead can do something it couldn’t do lying beside the urchin: block light from reaching the surface beneath it. Experiments on Moorea in French Polynesia found that these urchins covered more of themselves in bright conditions than in shade. The interesting part for a snorkeler is that you can watch the mechanics of making that shade, even when you can’t tell why a particular piece was picked up. 1 3

Follow one shell, not the whole pile

At first, the most conspicuous things in view may all belong to something else: a chip of coral skeleton, a piece of shell, a stray strip of weed. Look just below the lowest piece for the curve they’re resting on. The Australian Museum describes a typical pin-cushion urchin, another name for this species, as having short white spines, sometimes tipped orange. A glimpse through a gap in the cover is more useful than the color of whatever happens to be on top. Several species of urchin collect debris; a shell-covered dome alone won’t settle which one you’ve found. 1 2

If there’s movement, choose a fragment with a recognizable corner. Watch that corner against two references: a fixed point on the reef, and the rim of the urchin itself. If the whole dome travels and the corner keeps its place above the same patch of spines, the animal is carrying it. If the dome holds its position but the corner moves toward the crown, you’re watching the load change place on its carrier. A blade waving back and forth while its base stays put is a different sight again.

The best view of a pickup is often at the side. Researchers filming collector urchins in the Philippines recorded several flexible tube feet taking hold of a single piece of rubble. The feet pulled while spines underneath pushed; ahead of the piece, spines could lie down out of its way. In the close-ups you can distinguish a foot bending around an edge from a spine tipping under it. On the reef you might only see the result: a hard chip traveling up the side of a rounded animal without sliding back to the bottom. 1

A patch of shade that travels with the urchin

A ledge makes a useful comparison. Its shadow stays with the rock; a shell’s shadow can stay with the animal. On Moorea, a researcher compared collector urchins only partway under rocks with ones completely underneath. The partly exposed animals carried more cover. It’s a pattern you can look for from open water if both kinds of shelter are visible. There’s no need to peer deep under a ledge or turn a rock over to find its most sheltered resident. 3

Out on the reef, two animals beside the same ledge may also have different loose material within reach. To distinguish the effect of light from the luck of finding a shell, the Moorea study offered the same ten urchins pieces of plastic under bright and shaded conditions. After half an hour, the animals with intact spines had covered about 55 percent of their upper surface in the bright treatment, against about 39 percent in the dim one. Those are averages at the end of the trial, not a target that a wild urchin must reach before it stops collecting. 3

The clear pieces are the revealing detail. An urchin could lift one and hold it overhead just as it could an opaque red piece, but the two wouldn’t cast the same shadow. Under bright conditions the opaque pieces occupied more of the animals’ surface than the clear ones did. In dim conditions the two took up equal areas on average. That’s good evidence that blocking light matters. It isn’t evidence that these urchins always want red objects, or that a white shell on one wild animal tells you which wavelengths of light it’s responding to. 3